For acute angle θ, sec²θ = 1 - tan²θ

Assertion(A): sec²θ = 1 - tan²θ is not a trigonometric identity.

Reason(R): For an acute angle θ, the trigonometric equation is an identity, if it is satisfied for every value of angle θ.

1. A is true, R is false.

2. A is false, R is true.

3. Both A and R are true and R is correct reason for A.

4. Both A and R are true and R is incorrect reason for A.

The valid trigonometric identity;

sec²θ = 1 + tan²θ

So, assertion (A) is true.

A trigonometric identity is an equation that holds true for all values of the variable (in this case, the angle θ) within its domain.

So, reason (R) is true.

∴ Both A and R are true and R is correct reason for A.

Hence, option 3 is the correct option.